"Sputter etching" is to be understood to mean herein a method for the removal of material by ion bombardment. The material to be removed or the underlying substrate may itself serve as a cathode in a gas discharge or be cathodically biased; it is also possible to place the substrate with the layer or layers to be removed locally in the path of the bombarding ions of a gas discharge. Since the bombarding ions are impinging on the object with the material to be removed, in a substantially equal direction, the boundary of the etching will accurately follow the edge of the masking used. In this manner, a much more accurate boundary of layers to be etched away can in principle be realized than in chemical etching, in which the boundary can extend below the mask by underetching.
A difficulty in sputter etching, however, was to obtain a good mask which could be removed selectively after the etching treatment. For that purpose it was known, for example, to choose a material which was sputtered away at a comparatively low speed as compared with the layer or the system of layers which had to be given a certain pattern by sputter etching and which are soluble in a chemical etchant which did not attack the underlying layer to be masked locally. The masking layer was given the desired pattern by means of a thin layer of photoresist followed by a chemical etching treatment of the masking layer. In general the photoresist layer was then removed, although in special cases it could also be retained, since in the chemical etching treatment which was carried out to remove the masking layer after sputter etching a system of layers consisting mainly of silver any photoresist layer is lifted-off. By way of example, for the sputter etching of such a system of layers having a thickness of 1 .mu.m, a mask layer of aluminum with a thickness of 0.5 .mu.m was used which could easily be etched selectively by means of sodium hydroxide solution. The masking layer was comparatively thick in said known method. In chemical etching treatment of a pattern in said comparatively thick masking layer, a proportional extent of underetching is difficult to avoid so that the boundary of the sputter etching is less accurately defined.
It was also known that a photoresist layer itself was readily useful as a masking pattern for sputter etching. A difficulty was that photoresist layers which had been exposed to an ion bombardment could be removed with difficulty only afterwards. In order to be able to better remove such a photoresist layer which had been exposed to ion bombardment, it was known to use an intermediate layer of polyvinylformal directly on the masked parts of the layer to be removed locally by sputter etching. After sputter etching, the photoresist pattern with the underlying polyvinylformal layer were removed by an ultrasonic treatment in ethylene dichloride.
However, such an intermediate layer of polyvinylformal can be provided in a small thickness only with difficulty and will generally be at least approximately 1 .mu. thick. In order to obtain the photoresist pattern, a developer may be used which also dissolves the parts of the polyvinylformal layer exposed during the development, or said exposed parts should be dissolved in a separate step after the development. It is of importance that said local dissolving of the exposed parts should be done as completely as possible. Actually, organic polymers are generally resistant to sputter etching treatments. Remnants of the intermediate layer at the site where the photoresist has been removed may disturb or prevent a uniform sputtering away of the underlying layer or layers. A long lasting etching treatment on the contrary may result in a stripping off of the photoresist pattern or at least give a certain underetching of said pattern. In the latter case, a less accurate definition of the boundary of the sputter etching will be obtained due to the comparatively large thickness of the intermediate layer. Edge parts, if any, of the photoresist pattern projecting as a result of the underetching may actually sag during the sputter etching and fold onto the sides of the remaining parts of the intermediate layer. Furthermore it is possible that due to the ion bombardment and the released thermal energy the polymeric intermediate layer can also be removed with greater difficulty only.
One of the objects of the present invention is to provide a method which does not exhibit the above-mentioned drawbacks.
According to the invention, a method of the kind described in the preamble is characterized in that the intermediate layer consists of a thin layer of a metal which can be removed by sputter etching at a rate which is at least of the same order as the rate at which the material of the underlying layer to be removed locally can be removed by sputter etching or as the average rate at which the system of the underlying layers to be removed locally can be removed by sputter etching, the masking pattern comprising the photoresist being removed after the sputter etching treatment by selectively etching away the intermediate layer. A thin layer is to be understood to mean herein layers having thicknesses smaller than 0.5 .mu.m; the thickness of the intermediate layer is preferably at most 0.3 .mu.m. Such a thin metal layer would in itself not be suitable for use as a mask against sputter etching.
Where the sputter etching rate of the intermediate layer is compared here with the sputter etching rate of the underlying layer to be removed locally or with the average sputter etching rate of the system of layers to be removed locally, said comparison relates to the same methods of performing the sputter etching treatment. The average sputter etching rate of a system of layers is to be understood to mean herein the quotient ##EQU1## of said system of layers of n layers, wherein V.sub.p is the sputter etching rate of the material of the p.sup.th layer and d.sub.p is the thickness of the p.sup.th layer. Two rates of the same order is to be understood to mean herein that one rate is at least half and at most twice the other rate.
When, according to the preferred embodiment, the free parts of the intermediate layer are etched away selectively after the manufacture of the masking pattern from the photoresist and prior to sputter etching, a shortlasting etching treatment will suffice in view of the small thickness of the intermediate layer, in which underetching along the edges of the masking pattern can be restricted to a minimum. Possible sagging of the edge portions of the masking pattern is also a minimum so that a more accurate definition of the boundaries of the pattern to be manufactured can be achieved.
However, it is also possible, according to a further preferred embodiment, to omit the last-mentioned etching treatment of the free parts of the thin intermediate layer and to remove said free parts also during the sputter etching treatment so that an underetching effect need not be taken into account at all. In contrast with the known intermediate layer, the intermediate layer in the method according to the invention can easily be removed by sputter etching due to the material used therein and its small thickness.
Generally speaking, the invention may be used in those cases in which layers are to be provided in an accurately determined pattern. Such great accuracies are of importance in particular for those fields of technology in which great accuracies and layer parts with microscopically small dimensions are of importance, such as in optics and electronics, in particular in the manufacture of semiconductor devices, for example, integrated circuits. It is known, for example, to provide contact windows of very small dimensions as accurate as possible in an insulating layer, for example, a silicon oxide layer, present on a semiconductor surface, for example, of silicon. The method according to the invention is of particular importance to provide conductor patterns with conductor tracks having widths which are as accurate as possible and often as small as possible at very small mutual distances, in which the conductors should be given a reasonable thickness so as to keep the resistance of the conductors low. The remaining parts of the locally removed layer or the locally removed system of layers which consists of electrically conductive material and electrically conductive materials, respectively, may advantageously be used in semiconductor devices as a pattern of conductors which is at least partly present on an insulation layer. It will be obvious that for such applications the method according to the invention is to be considered in particular. It has proved possible, for example, with a thickness of the layer or the system of layers of at least 0.5 .mu.m, to provide a local gap with a width of at most 2 .mu.m.
In principle, the pattern of the layer or of the system of layers present one on top of the other can be removed locally throughout the thickness by sputter etching. It should be taken into account that the material of the substrate present immediately below the sputtered-away material may be exposed to the ion bombardment for a short time. As to how far such bombardment is permissible, should be considered for each individual case. In the case of the manufacture of a pattern of conductors on a semiconductor which is covered by an insulation layer, ions impinging upon said insulation layer may produce static charges which may cause undesired surface properties in the semiconductor, such as inversion channels and instabilities as a result of slow migration of said charges. Furthermore, the substrate surface may be sensitive to the etchant which is used to remove the intermediate layer and to lift the photoresist mask. In such cases, the sputter etching treatment may be continued in known manner until only a thin layer remains. This remaining thin layer may be removed by means of a short-lasting etching treatment in such manner that underetching is minimized.